1. Field of the Invention
The present invention relates to a position measuring arrangement for the determination of an absolute position. The present invention furthermore relates to a method for operating an absolute position measuring arrangement.
2. Discussion of Related Art
Position measuring arrangements in the form of angle- and length-measuring arrangements are employed in the machine tool industry and other manufacturing, handling and testing systems. A high functional dependability becomes of increased importance in connection with all uses, because faulty operation can cause considerable damage.
Absolute position measuring arrangements, which can provide correct position information in any relative position, even immediately following an interruption in the energy supply, are being increasingly employed for position measuring. In this case, the absolute position is represented by a single-track code arranged in a particularly space-saving manner of code elements arranged one behind the other in the measuring direction. Here, the code elements are arranged behind each other in a pseudo-random manner, so that a definite number of successive code elements respectively constitute a code pattern or bit pattern, which definitely defines the absolute position as a code word. A new code word is already formed in the course of the displacement of the scanning device by a single code element, and a sequence of different code words is available over the entire measuring range to be recorded. Such a sequential code is called a chain code or pseudo-random code.
A number of measures for assuring the dependable read-out of the code elements have become known in the prior art. These measures have in common that it is intended to assure that the code elements are scanned within the definite range, i.e. not in the transition area to the adjacent code elements. A selection information is obtained for this from an auxiliary track, or from the code track itself, by which the scanning elements are selected for assured scanning and generation of a code word. The binary information from the selected scanning elements is provided to a decoding device for forming the instantaneous absolute position. Stored assignment tables or generators are used as decoding device.
Such position measuring arrangements and methods are described, for example, in DE 42 09 629 A1, DE 39 42 625 A1, DE 43 09 863 C1, DE 38 25 097 C2 and WO 03/060431 A1.
It is customary to employ redundant scanning units for increasing the operational dependability and for detecting faulty operation of absolute position measuring arrangements. Such a position measuring arrangement is described in EP 0 789 226 B1. Several spaced apart code words are scanned and compared with each other. The hardware outlay is increased by this multiple redundancy, and connected therewith as the costs as well.